(1) Field of the Invention
The present invention relates to an improvement in a pulse electric power unit provided with a direct current power converter for converting an output voltage from a direct current power supply into a direct current voltage having arbitrary magnitude.
(2) Description of the Related Art
FIG. 1 is a circuit diagram showing the structure of a pulse electric power unit of a type wherein a Cuk converter employed as a conventional direct current power converter is pulse-operated to output pulse current. In the same drawing, there are shown a direct current power supply PS1, reactors L1, L2, a capacitor C1, a diode D2, a transistor TR1, a gate drive circuit GA1, a load RL, an output voltage V1 of the direct current power supply PS1, an output voltage V2 of the load RL, currents 1 through 4, and discharge current 19 of the capacitor C1. In FIG. 1, a description will now be made in a case where the transistor TR1 is a MOS (Metal-Oxide-Semiconductor) field-effect transistor (MOSFET) by an illustrative example. However, other semiconductor switching devices such as an IGBT (Insulated-gate Bipolar Transistor), an electrostatic induction type transistor, an electrostatic induction type thyristor, a GOT (Gate turn-off) thyristor, etc. will be described in the same manner as referred to above.
A description will now be made of the operation of the conventional pulse electric power unit shown in FIG. 1. When the transistor TR1 whose conductivity is controlled by the gate drive circuit GA1 is turned on, the current 1 indicated by the arrow flows through the reactor L1 connected to the direct current power supply PS1. On the other hand, when the transistor TR1 is turned off, the current 1 indicated by the arrow is switched to the current 2, which in turn charges the capacitor C1 and flows into the diode D2.
When the transistor TR1 is turned on again, the current 1 is further increased, so that the current 3 indicated by the arrow flows into the transistor TR1 through the reactor L2 and the capacitor C1. At this time, the current 3 flows into the load RL, so that the power conversion is effected.
Further, when the transistor TR1 is turned off, the increased current 1 is changed to the current 2, which in turn charges the capacitor C1. In addition, the current 3, which flows through the reactor L2 is changed to the current 4 indicated by the arrow, and the current 4 flows into the load RL in succession. The Cuk converter is operated in the above-described manner to effect direct current power conversion. When the Cuk converter is in operation, the capacitor C1 is charged so as to be the polarity shown in FIG. 1. The voltage stored in the capacitor C1 becomes equal to the sum of the output voltage V1 of the direct current power supply PS1 and the output voltage V2 applied across the load RL.
FIG. 2 is a block diagram showing the structure of a control circuit for pulse-operating the Cuk converter of FIG. 1. In the same drawing, there are shown a triangular-wave signal generator 5, a command control unit 6, a pulse signal generator 7, switches 8, 9, an inverter 10, a comparator 11, a gate drive circuit GA1, signals 12 through 14, and a pulse signal 24.
FIG. 3 is a timing chart for describing the operation of the control circuit of FIG. 2 for pulse-operating the Cuk converter, the chart showing signals employed in respective parts of the control circuit. In the same drawing, designated at numerals 12, 13a, 13b, 14 are signals and numeral 24 indicates the pulse signal.
In the control circuit shown in FIG. 2, the triangular-wave signal generator 5 generates a triangular-wave signal 12 shown in FIG. 3(b). The command control unit 6 produces a voltage corresponding to the conductivity of the transistor TR1 (see FIG. 1). The pulse signal generator 7 generates the pulse signal 24 shown in FIG. 3(a), which in turn actuates the inverter 10 to thereby open or close the switches 8, 9 alternately. The comparator 11 compares the signal from the triangular-wave signal generator 5 with the signal 13 from the command control unit 6 and outputs the signal 14 for making conductive the transistor TR1 to the gate drive circuit GA1. The signal 14 to be outputted to this gate drive circuit GA1 will be shown in FIGS. 3(c) and 3(d). FIG. 3(c) shows the signal 14 obtained when the command control unit 6 outputs the signal 13a depicted in FIG. 3(b), while FIG. 3(d) illustrates the signal 14 obtained when the command control unit 6 outputs the signal 13b shown in FIG. 3(b). However, when the level of the signal outputted from the command control unit 6 is reduced, the conductivity of the transistor TR1 becomes small so that the output power to be delivered to the load RL is lowered.
Since the pulse electric power unit employing the conventional Cuk converter is constructed as described above, such problems as will be described subsequently exist.
FIG. 4 is a timing chart for comparing the operation of the pulse electric power unit employing the conventional Cuk converter with that of a pulse electric power unit according to one embodiment of this invention to thereby describe the comparison result, the chart showing signals employed in individual parts of both power units. In the same drawing, designated at numerals 15, 18, 20 are output currents of the transistor TR1 and numerals 16, 17, 21, 22 indicate voltages stored in the capacitor C1. Numeral 23 indicates output current which flows into the load RL.
FIG. 4(a) shows an operation characteristic obtained when the transistor TR1 is turned on and off. When the pulse signal generator 7 shown in FIG. 2 is turned on to output an ON signal (the pulse signal 24) therefrom, the switching operation of the transistor TR1 is made in response to the signal shown in FIG. 4(a). As a consequence, the output current 15 from the transistor TR1 is increased as illustrated in FIG. 4(b). Further, a voltage stored in the capacitor C1 rises up to the voltage 17 as depicted in FIG. 4(c). Thus, the time required to charge the capacitor C1 is equal to the time T1 required for the output current 15 of the transistor TR1 to rise. The transistor TR1 should have the switching operations of 5 to 20 times in order to charge the capacitor C1. Therefore, the rise time T1 of the output current 15 becomes slow or long.
On the other hand, when the pulse signal generator 7 is turned off to output an OFF signal therefrom, the switching operation of the transistor TR1 is stopped in response to the signal shown in FIG. 4(a). As a result, the output current 18 of the transistor Tr1 is reduced as shown in FIG. 4(b). With this operation, a voltage stored in the capacitor C1 is reduced to the voltage 16 from the voltage 17 as illustrated in FIG. 4(c). This results from the fact that the switching operation of the transistor TR1 is stopped and at the same time, discharge current 19 indicated by the arrow (broken line) in FIG. 1 flows, so that the voltage stored in the capacitor C1 is discharged. The irregular or improper current 20 occurs as shown in FIG. 4(b) because the discharge current 19 flows through the load RL. Such improper output current 20 shows an output-current waveform different from that of the signal 13 outputted from the command control unit 6.
FIG. 5 is a timing chart for describing the operations effected when the pulse electric power unit shown in FIG. 1 is practically actuated, the chart showing signals employed in respective parts of the pulse electric power unit. In the same drawing, designated at numeral 24 is a pulse signal. Numerals 25, 27 designate voltages stored in the capacitor C1 and numerals 26, 28 indicate currents which flow into the load RL. At this case, a voltage 25 necessary to charge the capacitor C1 with respect to the pulse signal 24 shown in FIG. 5(a) is shown in FIG. 5(c) under the condition of operation that a frequency of a signal 12 from a triangular-wave signal generator 5 is 20 KHz, the inductance of each of the reactors L1, L2 is 100 .mu.H, the capacitance of the capacitor C1 is 100 .mu.F, the output voltage of the direct current power supply PS1 is 230 V, the resistance of the load RL is 1.5 .OMEGA., and the electric conductivity of the transistor TR1 is 50%. The time required to charge the capacitor C1 at the time that the voltage 25 rises is about 0.5 msec. As shown in FIG. 5(b), the output current 26 at the load RL also rises in the same manner as described above. The voltage 27 stored in the capacitor C1 at the time that it falls is shown in FIG. 5(c). At this time, the output voltage of the direct current power supply PS1 is reduced to 230 V. At the same time, improper proper current flows into the load RL owing to the presence of the improper output current 28 shown in FIG. 5(b) which flows in the load RL.
In order to adopt the Cuk converter as being used to illuminate an arc lamp for the YAG laser excitation or to perform an electric discharge between an electrode on the side of a discharge machining apparatus and an object or workpiece to be machined, the Cuk converter should have a high-speed response characteristic. Further, when the improper current flows into the load, the normal power of a laser beam cannot be produced. Furthermore, the converter fails to produce a normal discharge between the electrode and the workpiece side.
Accordingly, the conventional Cuk converter is unfit for the pulse electric power unit to illuminate the arc lamp for the YAG laser excitation or for the pulse electric power unit to supply the proper current to the electrode-workpiece interval. Thus, such a Cuk converter has not been put to use.
As has been described above, the above-described Cuk converter is accompanied by a problem that inconvenience such as a slow rise in output current from the Cuk converter and occurrence of improper output current at the time that the output current falls is developed. The Cuk converter is accompanied by another problem that the response speed is slow where it is employed as a pulse electric power unit necessary to have high-speed response as particularly in the pulse electric power unit used to illuminate the arc lamp for the YAG laser excitation or in the pulse electric power unit used to provide the proper current between the electrode on the side of the discharge machining apparatus and the workpiece to be machined.